This invention relates to the extraction of elemental sulphur from sulphur compound gases and particularly to the extraction of elemental sulphur from the hydrogen sulphide present in natural gas both for the purpose of obtaining sulphur and for the purpose of disposing of the hydrogen sulphide.
Hydrogen sulphide is often a component of natural gas as extracted from the field and for environmental reasons this can not be allowed to remain in the natural gas when it is supplied to consumers. It has therefore been a practice for many years to extract the hydrogen sulphide and to dispose of the hydrogen sulphide by reacting it with sulphur dioxide to produce elemental sulphur and water. This reaction takes place at a practically acceptable extraction level only in the presence of a catalyst such as finely divided alumina.
In recent years it has been found that higher extraction levels can be obtained by using catalyst at a temperature less than the dew point of sulphur. This causes the sulphur to condense onto the catalyst thus gradually poisoning the catalyst. It is necessary therefore periodically to regenerate the catalyst. This regeneration can be carried out by applying the hot supply gases of hydrogen sulphide and sulphur dioxide to the catalyst at a temperature above the dew point so the sulphur is evaporated off the catalyst for collection in condensers downstream of the catalyst. Thus systems have been set up using two or more beds of the catalyst in which one bed is being regenerated by the evaporation and collection of the sulphur while another bed is at a lower temperature so that the reaction takes place at a high level of extraction. Periodically, typically daily, the beds are reversed in order relative to the path of the gases so that the first bed is used for the reaction while the second is being regenerated.
U.S. Pat. No. 2,767,062 (Duecker) and U.S. Pat. No. 3,749,762 (Montgomery) discloses in FIGS. 1 and 2 a system of this type. The system has a number of considerable disadvantages in that while the process is nominally a continuous process there is a considerable time of the order of 11/2 minutes in which switching is taking place in which the recovery is very poor and hence sulphur gases are emitted to the atmosphere and the longer period of time where the beds of catalyst are at the wrong temperature and hence operating inefficiently.
In addition there is considerable disadvantage in that the beds must be of a very large size in order to accommodate the amount of gas and in a practical example for a plant having 250 LTD capacity requiring 99% sulphur recover the beds in total may amount to 600 tons (545 tonne) with consequent high capital cost in installation.
Furthermore, the switching between the beds causes a heat/cool cycle in the ducting and in the supports for the beds with consequent expansion and contraction problems which must be accommodated at considerable expense in the design.
A yet further disadvantage is that the actual switching mechanism provided by valves are individually of considerable expense and a number of such valves are required in the construction. In practical examples three or more beds are used rather than the two beds proposed by Duecker with each bed being associated with its own condenser in order to reduce the numbers of valves used as in U.S. Pat. No. 3,749,762. This requirement to associate a particular condenser to a particular bed means that the condenser has to be designed in an attempt to accommodate the different parameters of the different functions thus reducing the efficiency and increasing the capital cost by the necessary compromise.
Duecker, in FIG. 4, also proposes a continuous system using two moving beds of the catalyst so that the catalyst moves from the reactor directly to the regenerator in a continuous stream. This arrangement has a number of problems and certainly has never become accepted in the field and it is believed that it has never been practically manufactured.